vmm: refactored lib.rs tests
vmm: refactored lib.rs tests

In all tests we were initializing in the same way the kernel.
Addded a method in the test implementation of Vmm called
default_kernel_config.

Signed-off-by: Andreea Florescu <f68a7d732e796861f8e79ea704b43a5ee52eddaf@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker